Changes in Pasteurized Donor Human Milk during Refrigeration

Dear Editor, We have attentively read the Vickers et al article about storage of pasteurized donor human milk (PDHM) at 4oC for 96 hours. In this study, its authors resolve that there was no evidence of microbes in PDHM that had been defrosted and stored at 4oC for up to 9 days. Previously, Slutzah et al had shown how bacterial growth in fresh milk remained unchanged for 96 hours and, therefore, concluded that fresh mother’s milk may be stored at refrigerator temperature for as long as 96 hours. However, they also showed how fresh milk’s pH significantly decreased during this refrigeration period (from 7.21 to 6.68 over 96 hours of storage), which relates to the increase in free fatty acids. Although fresh milk appears to be safe after 96 hours of refrigeration, the acidification that takes place in this process may alter its quality. Acid milk seems to hinder calcium absorption; its proteins become thermosensitive and changes in the milk’s cellular content have also been described. In this same article, apart from 36 fresh milk samples, Slutzah also analyzes pH evolution in 5 samples of PDHM during refrigeration and points out that it barely suffers any modifications (pH of 6.3 ± 0.1). Our team analyzed changes in pH in 30 PDHM samples stored at 4oC for 96 hours, and we observed that the pH hardly varies (from 7.56 to 7.58). These results, together with those of Vickers, suggest that PDHM stored at 4oC for up to 96 hours remains safe regarding bacterial growth and quality due to pH stability. However, our attention is drawn to the low pH values for PDHM reported in Slutzah’s article in comparison to the ones that our team obtained. The average pH values reported for fresh milk range from 7.68 to 7.07 with a median of 7.56, which are very similar to those obtained in our study. In a previous research study, it was also established that the minimum acceptable pH for subsequent processing of milk in the bank would be 6.57. The pH that Slutzah reported for PDHM is below this limit. In breast milk, pH is easy to check, but a fixed criterion on how it varies over time and on the effects of this variation remains to be established. This means that more studies concerning this issue would be desirable in order to learn more about the consequences of a pH decrease in milk’s quality. Its preservation is a very important task and a technique as simple as pH measuring could possibly be helpful in this regard.